Cooking Time Scaler

A Guide to Adjusting Cooking Times

The Cooking Time Calculator estimates adjusted cook times and temperatures when changing recipe size or switching between oven, slow cooker, instant pot, and air fryer methods.

A thermometer in the centre of the food is the only thing that tells you it is done. Every number this tool produces is a planning estimate: it tells you when to start checking, not when to stop cooking. Use it to schedule the oven, then let internal temperature decide the finish.

With that established, the timing question is worth understanding, because the intuitive answer is wrong twice over. Cooking time does not double when food size doubles — but nor does it rise by the modest amount a cube-root rule suggests. In 1822, the French mathematician Joseph Fourier published Théorie analytique de la chaleur, the work that still governs how heat moves through solids. Fourier's own result is the one that matters here: two similar bodies reach the same condition "after times which are as the squares of the dimensions". Squares, not the dimensions themselves. Doubling a roast's weight makes it about 26% thicker, and conducted heat needs time proportional to the square of that distance — so the time rises by about 59%, not 26%. Getting this wrong in the low direction is the dangerous mistake: it leaves scaled-up meat underdone.

Cooking Method Comparison

Different appliances transfer heat at different rates, which is why a dish that takes one hour in a conventional oven does not take one hour in a slow cooker or air fryer. The table below compares the four methods this tool supports.

These multipliers are starting points derived from USDA guidelines and manufacturer recommendations. Actual results vary by appliance brand, food density, and starting temperature. A frozen roast placed directly in a slow cooker will take longer than one that started at room temperature.

How the Calculator Works

Enter the original cook time and temperature, the size change factor (2 means doubled, 0.5 means halved), and the target cooking method. For oven and air fryer the tool raises the size factor to the two-thirds power — adjusted time = original time × (size factor)^(2/3) — then applies the method-specific multiplier and temperature adjustment. For slow cooker and pressure cooker it applies no size exponent at all, for the reason given below. For a full recipe scaling walkthrough that covers both ingredient and time adjustments, the CookCalcs scaling guide provides a step-by-step process.

Why the Exponent Is Two-Thirds, Not One-Third

Heat conducts inward from the food’s surface, and the time it needs grows with the square of the distance it has to travel. That squaring is the step intuition skips. For a roughly spherical object — a meatball, a compact roast — doubling the volume increases the radius by ³√2 ≈ 1.26, so heat must travel 26% further. Squaring that gives 1.26² ≈ 1.59: the cooking time rises by about 59%. Because thickness itself grows as the cube root of mass, the rule expressed in weight is that time scales with weight to the two-thirds power.

Published roasting tables agree. Fitting a power law to the USDA Food Safety and Inspection Service turkey and ham timetables, and to Food Standards Scotland's per-kilogram rules, gives exponents clustered between roughly 0.5 and 0.7 — around two-thirds, and nowhere near one-third. The table below shows what the difference costs on a real roast.

The errors are not symmetrical in consequence. Scaling down and overshooting gives dry food; scaling up and falling short gives underdone meat, which is a food-safety problem rather than a quality one. That asymmetry is why the tool now uses the exponent the conduction physics and the published tables both point at. The model still works best for solid, uniform proteins and dense baked goods, and least well for thin flat foods, where surface area dominates, and for hollow items such as stuffed peppers or bread loaves.

For baked goods that change container shape when scaling, the bake time depends on batter depth rather than total volume. The bake time adjustment tool for different pan sizes handles that geometry specifically. For dishes being scaled in ingredient quantity, combine the scaling factor with this time adjustment for a complete picture.

Slow Cooker Conversions

Slow cookers maintain a steady temperature between 170 °F (low setting) and 200 °F (high setting), relying on long exposure and trapped moisture. This tool uses the 5× multiplier at 200 °F: a 60-minute oven dish becomes approximately 300 minutes (5 hours) in the slow cooker. The trade-off is texture — slow cooking breaks down collagen in tough cuts, producing fork-tender results that a one-hour oven roast cannot achieve. That same collagen breakdown makes slow cookers ideal for brined roasts and large cuts that benefit from extended low-temperature cooking.

Notice that the size factor does not change the slow cooker figure. That is deliberate, and it is worth explaining rather than hiding. No food-safety authority publishes a rule for scaling slow cooker time with batch size — not the USDA, whose fact sheet governs the heat setting and the cooker size but never the quantity, and not the university extension services. The numbers circulating online for this ("add half an hour per doubling" and similar) trace to recipe sites with nothing behind them, so this tool declines to invent one. What the authorities do publish is a limit on fill: no less than half full and no more than two-thirds full, which extension guidance explains is what assures even heat distribution. Read that as the real constraint — if a doubled batch would overfill the crock, the answer is a bigger cooker or two cookers, not a longer guess. An over-full cooker heats its contents more slowly, which is the direction that matters, since food lingering between 40 °F and 140 °F is where the risk lives.

One citation note, because it is easy to get wrong: the half-to-two-thirds line comes from the 2009 revision of the USDA fact sheet. The current version dropped it and points readers to their appliance's own instruction booklet instead, so anyone quoting it should cite the earlier revision or the extension services that still carry it.

Instant Pot and Pressure Cooking

Pressure cookers raise the boiling point of water to approximately 250 °F by maintaining 10–15 psi of pressure. Food cooks in roughly one-third the oven time. A stew that takes 90 minutes in the oven needs about 30 minutes under pressure, plus time for the cooker to reach and release pressure (typically 10–15 minutes each). The temperature output shows 0 °F for pressure cooking because the temperature is not user-configurable — it is a fixed property of the pressure level.

Pressure cooking is the one method where the scaling question has a published answer, and it surprises most cooks: quantity does not change the time at pressure. Presto states it plainly in its own manuals — the quantity of vegetables does not change pressure cooking times — and adds that for larger cuts, thickness rather than weight is what determines the time. Its single published adjustment is a thickness one: roasts thicker than three inches get five extra minutes. Fissler describes its timings the same way, by the size of the pieces rather than the size of the batch. So a doubled recipe means doubled ingredients and the same cook time. What does grow with a fuller pot is the overhead at either end: coming up to pressure takes longer, and a natural release can run to forty minutes or more depending on how much food is in the cooker. Budget for that, not for a longer cook.

Air Fryer Adjustments

Air fryers are essentially small convection ovens with powerful fans. The rapid air circulation browns food quickly and cooks it about 20% faster than a standard oven. The oven temperature converter can help translate the adjusted air fryer temperature between Fahrenheit and Celsius. Because air fryer baskets are small, large-batch cooking requires multiple rounds, and crowding reduces efficiency by blocking airflow.

Limitations

Two-thirds scaling assumes a food that keeps roughly the same shape as it grows, and a consistent starting temperature. Irregularly shaped foods, hollow items (stuffed peppers, bread loaves), and frozen-to-cooked transitions do not follow it precisely. It also assumes the size change is a bigger piece: two roasts side by side in one oven are not one roast of twice the weight, and cook in close to the original time. For baked goods that change container shape, bake time tracks batter depth rather than total volume, which the bake time adjustment tool for different pan sizes handles directly. For holiday meals where multiple dishes share oven time, the holiday cooking coordination guide offers strategies for staggering items. This tool does not account for altitude, which lowers the boiling point of water and affects both pressure cooking and oven baking at elevations above 3,000 feet.

Key Terms

Two-Thirds Scaling

The rule that cooking time rises with the two-thirds power of the size ratio for conduction-cooked food. Doubling the weight increases time by a factor of 2^(2/3) ≈ 1.59, or about 59%. It follows from conducted heat needing time proportional to the square of the distance travelled, combined with thickness growing as the cube root of mass.

Convection

Heat transfer by moving air or liquid. Air fryers and convection ovens use fans to circulate hot air rapidly around food, transferring heat more efficiently than the still air in a conventional oven. This is why air fryers cook faster at lower temperatures.

Safe Processing Times

For preserved and canned foods, processing times are safety-critical and must not be shortened by method conversion. Canning times are established through laboratory testing and should never be adjusted using a general cooking time calculator.